1. Field of the Invention
The present invention relates to a disk drive apparatus for use with a disk-shaped information recording medium, such as a magnetic disk drive apparatus, an optical disk drive apparatus, or the like, a head positioning system of the disk drive apparatus, and a head positioning method for the disk drive apparatus.
2. Description of the Prior Art
In recent years, a demand of the market for a high recording density disk drive apparatus, in/from which a large amount of data, such as video information, sound information, character information, or the like, are stored/read at a high speed, has been increasing along with the progress of multimedia industries. Among various disk drive apparatuses, a magnetic disk drive apparatus is small and less expensive but has a large capacity, and is capable of high speed data transfer. Applications of such a magnetic disk drive apparatus have been increasing not only in uses for personal computers (PCs), but also in the AV (Audio-Visual) industry and car electronics industry. Moreover, the need for magnetic disk drive apparatuses in the so-called mobile communication industry typified by terminal devices for mobile communication has been increasing, and therefore, a further reduction in size of magnetic disk drive apparatuses has been demanded. Among applications other than the applications to PCs, the other advantages of magnetic disk drive apparatuses, i.e., small size, small power consumption, and vibration/impact resistance, as well as high density (large capacity), are considered to be important. Thus, a demand for precise and quick positioning of a head to a target position has become more stringent along with an increase in the recording density.
In general, a positioning mechanism using an actuator is used as means for positioning a head of a disk drive apparatus on a recording medium. Generally known positioning mechanisms are a linear actuator and a rotary actuator. Both the linear actuator and the rotary actuator are guided by a ball and roller bearing. However, when the actuator is driven for moving a head supporting mechanism with such a bearing, friction force which is a force against the movement of the head supporting mechanism always occurs in the bearing. For example, when the head supporting mechanism starts to drive, the actuator must have a driving force that exceeds the friction force resulting from the static friction between the bearing and the head supporting mechanism. After the movement of the head supporting mechanism is started, the friction force resulting from the kinetic friction occurs between the bearing and the head supporting mechanism. In general, when a movable part like such a head supporting mechanism is moved, the static friction is larger than the kinetic friction. Thus, when the movement of the head supporting mechanism is started, a larger driving power is required. Thus, in a mechanism that moves using a bearing, smooth movement is difficult to achieve because of the difference between the static friction and the kinetic friction, and the servo control for positioning the head supporting mechanism may not be precisely performed.
Furthermore, as the size of the disk drive apparatus is decreased, the size of the bearing is accordingly decreased, so that the influence of such friction forces on the movement of the head supporting mechanism becomes larger. Furthermore, the size and weight of the head supporting mechanism are also decreased, and therefore, reaction force caused by a flexible print circuit (FPC), which is connected to a head and transfers an electric signal thereto, largely influences the movement of the head supporting mechanism as well as the friction force. Thus, as the size of the disk drive apparatus is decreased, the friction force in the bearing, the reaction force by the FPC, and actuator vibration produced by the spindle vibration caused due to rotation of a disk is more likely to adversely influence the performance of the disk drive apparatus.
In magnetic disk drive apparatuses, or the like, position information are written in advance on a disk in many cases, but the signals obtained from the position information are discrete signals. Moreover, the number of pieces of position information is limited because sufficient data regions should be secured for writing new information on the disk. In a small size disk drive apparatus, there is a limit to an increase in the number of revolutions due to restrictions on the system specifications. Thus, it is impossible to sufficiently increase the sampling frequency. As a result, the increase of the control frequency is restricted. This also can be a factor that adversely influences the reduction in size and increase in recording density of the disk drive apparatus.
As the size of the disk is reduced and the recording density of the apparatus is increased, the friction in the bearing more largely influences the control of the driving operation of the positioning system with the actuator. This influence is not negligible in the positioning of the head. Conventionally, against such various factors that cause a decrease in the positioning precision, the following countermeasures have been proposed: (1) a mechanism for reducing disturbance such as friction; (2) a method for estimating and compensating for friction and disturbance vibration by an observer in the control process; and (3) a method for suppressing disturbance by a high band control.
For example, as means for mechanically reducing disturbance (countermeasure (1)), a method for using a member having desirable slidability in the bearing and a method for suppressing static friction with kinetic friction have been known. As a method for estimating and compensating for friction by an observer (countermeasure (2)), the load control performed based on continuous signaling using a counter electromotive voltage signal of an actuator (VCM) has been proposed. However, in the case where the counter electromotive voltage signal is used as a control signal, a variation in the resistance value due to a variation in the coil resistance, a temperature variation, etc., influences the controllability. Thus, various correction methods have been proposed. As for the high band control for the countermeasure (3), there is an example where an acceleration sensor signal other than a discrete position signal is used.
Hereinafter, some specific propositions for the countermeasures (1) and (2) are exemplified and briefly described, while the countermeasure (3) which is not much relevant to the present invention is not herein described.
A proposed example of a method for suppressing an influence of friction (countermeasure (1)) is a method for driving a head to incessantly tremble. Specifically, the disk is incessantly wobbled with respect to a head such that a kinetic friction state always occurs between a head supporting mechanism and a bearing, whereby an influence of static friction is eliminated (see, for example, Japanese Unexamined Patent Publication No. 10-172229).
FIG. 17 illustrates an operation principle of a head positioning system of a conventional disk drive apparatus. Specifically, FIG. 17 shows a structure of a rotation controlling mechanism which utilizes wobbling. Herein, illustration of the entire structure of the disk drive apparatus is omitted and only the rotation controlling mechanism, which is a key element of the head positioning system, is described.
In FIG. 17, a disk 311 is placed on a disk table 393 and rotates according to the rotation of a motor shaft 394. The disk 311 has a large number of tracks 396 which are concentrically formed around the center of the disk 311. The head supporting mechanism having a head mounted thereon is driven by an actuator. The head is moved over the disk 311 to a target position by controlling the actuator. The head writes/reads information on/from a track at the target position. The disk table 393 has a disk shape and has a chucking positioning section 395 at the center thereof. The chucking positioning section 395 fits in the center hole of the disk 311, thereby positioning the disk 311. A portion of the disk table 393 is provided with a counter weight 397.
In general, a disk, a disk table and a motor shaft are provided in a concentrical configuration. However, in this proposed example, the center of the disk 311, i.e., the chucking center CC, is eccentric from the central axis of the disk rotation control, i.e., the motor shaft axis CM, by an eccentric amount (distance) d as shown in FIG. 17. In such a structure, when the disk 311 is rotated, the head incessantly reciprocates (i.e., wobbles) along the radial direction of the disk 311 according to the eccentric amount d. That is, friction force resulting from kinetic friction always resides between the head supporting mechanism and the bearing, and accordingly, friction force resulting from static friction rarely occurs therebetween. Thus, the operation of moving the head mounted on the head supporting mechanism is smoothly and precisely performed, and positioning of the head is precisely realized.
Furthermore, as described above, the disk table 393 is provided with the counter weight 397. The counter weight 397 is attached on the lower surface of the disk table 393 and positioned on the opposite side to the chucking center CC with respect to the motor shaft axis CM, such that the center of gravity of the entire structure resides on the motor shaft axis CM. Thus, the vibration caused due to eccentricity when the disk 311 is rotated is suppressed. In this proposed example, the above structure suppresses an adverse influence of static friction on the head positioning operation.
As a method for estimating and compensating for disturbance by an observer (countermeasure (2)), an exemplary control method applied to the load control operation has been known, although the exemplary control method is not the control for improving the positioning precision, such as a following control (see, for example, the spec of Japanese Unexamined Patent Publication No. 11-25626). In this example, when the control is performed using a counter electromotive voltage signal of the VCM, an estimation error in the velocity estimation due to the counter electromotive voltage signal is corrected. Specifically, before the load control operation, the dynamic range of the counter electromotive voltage signal and the offset that occurs when the counter electromotive voltage signal is converted by an AD converter and input to a CPU are corrected. Then, the velocity control is performed such that the slider does not collide against the disk and the head is stably moved over the disk.
FIG. 18 shows a structure of a control operation mechanism provided in the above-described exemplary disk drive apparatus which performs the load control operation using the counter electromotive voltage signal of the VCM. FIG. 18 further shows a flow of the load control operation. In FIG. 18, the disk drive apparatus 410 includes: a VCM spindle motor driver 412 for rotating a disk 411; a magnetic head 413; an actuator 414 for guiding a head slider which has the magnetic head 413 onto the disk 411 or moving the head slider to a retreat position; a ramp 415; and a CPU/HDC 416 for performing the velocity control of the actuator 414; the calibration control for detecting the offset and dynamic range of an AD converter prior to the velocity control, the control of write/read operations, etc.
In the disk drive apparatus 410, when the magnetic head 413 is loaded from the ramp 415 on the disk 411, the velocity is estimated from the counter electromotive voltage signal of the VCM to perform the velocity control. Counter electromotive voltage detection means of the CPU/HDC 416 includes a bridge circuit for detecting as the counter electromotive voltage the voltage caused in the coil by balancing the coil resistance of the VCM with a predetermined resistance. The resistance value of the bridge circuit is balanced based on the coil resistance value obtained when the coil is at a room temperature. In the example illustrated herein, prior to the control operation, during when the head is on the ramp, a voltage that moves the actuator in the opposite direction is applied so that the head is pushed against the ramp. The dynamic range of the voltage output from the head pushed against the ramp and the offset of the AD converter are calibrated.
Furthermore, in an example of a proposed method for correcting an estimation error of the velocity estimation which is caused due to a variation in the temperature of a coil, the relationship between the velocity estimation value, which is estimated based on the counter electromotive voltage signal, and the detected voltage is corrected according to the state where an actuator is pushed against a stopper before the load control operation is performed and the state where the actuator is pushed against an inner periphery stopper before the unload control operation is performed (see, for example, Japanese Unexamined Patent Publication No. 2000-163901).
FIG. 19 is a flowchart of a control process of a disk drive apparatus that uses the above method.
In a disk drive apparatus control system of FIG. 19, a calibration operation is performed at the start of the load control operation of loading the head from the ramp onto the disk. In the calibration operation, a VCM velocity detection value, which is detected by a VCM velocity detector while the actuator is pushed against the outer periphery stopper such that the actual velocity of the VCM is zero, is read and a velocity correction value used for correcting the relationship between a VCM current value and the VCM velocity detection value is obtained based on the detected VCM velocity detection value. During the head positioning period that occurs after the loading operation, the above-described calibration operation is performed again while the actuator is pushed against the inner periphery stopper at periodic intervals counted with a timer in order to update the velocity correction value, and the head is returned to an original head position.
However, in the disk drive apparatus of FIG. 17 which utilizes wobbling, an eccentric mechanism is incorporated in the mechanical structure of the apparatus in order to suppress the influence of the static friction. Thus, there is a possibility that the influence of vibration on external elements due to the eccentric disk rotation mechanism becomes normegligible. In this proposed example, a special production method and special elements are necessary for the eccentric structure additionally to those required in a general disk drive apparatus having a non-eccentric mechanical structure.
In the proposed example of FIG. 17, the motor shaft axis CM and the chucking center CC are deviated from each other. Thus, the driven disk table and the disk are rotated in an eccentric state with respect to the rotation of the motor shaft, and accordingly, vibration that is in synchronization with the number of revolutions of the disk table and the disk is likely to occur in the entire structure of the disk drive apparatus. Such vibration produces noise to the outside and should be avoided in an appliance that uses the disk drive apparatus. Furthermore, there is a possibility that vibration occurs in a direction perpendicular to the disk. The vibration of such a direction may be a factor to cause a contact of a head with a disk in a magnetic disk drive apparatus that uses a floating magnetic head. In order to avoid such a problem, in the structure shown in FIG. 17, the counter weight is added such that the center of gravity is on the motor shaft axis CM. However, in such a method wherein the problem is solved by adjusting a balance, it is necessary to precisely adjust the weight and position of the counterweight, and the efficiency in the production process is deteriorated. Furthermore, size reduction of the disk drive apparatus requires more precise balance adjustment. Further, influence of the reaction force of the FPC is also non-negligible, and the balance of forces must be considered. In the method described in this example, vibration readily occurs due to the eccentric rotation, and therefore, suppression of the vibration is more restricted as the size of the disk drive apparatus decreases.
The exemplary structure shown in FIG. 17 is different from a generally-employed structure where the motor shaft, the disk table and the disk are placed in a concentrical configuration. Thus, in the realization of a disk drive apparatus having the conventional structure, a special mold and elements are required. Furthermore, in the production thereof, it is necessary to provide a special balance adjustment step as described above. Thus, there is a possibility that the price of a product finally increases due to the necessity for special elements and a decrease in the efficiency of the production process.
In the disk drive apparatus whose general structure is illustrated in FIG. 18 and the disk drive apparatus whose control system process is illustrated in FIG. 19, the counter electromotive voltage signal is used only as a velocity feedback signal in the load control operation for loading the head. In these disk drive apparatuses, if the velocity estimation value obtained from the counter electromotive voltage signal is used in the following control operation, the following three problems occur.
The first problem is that an error occurs in the estimated velocity if the same correction value is used in the load control operation and the following control operation, because the current value for the control driving is different between these operations by a factor of 10 or greater, and the resistance value of the coil changes according to the temperature characteristic of the coil resistance. In the proposed example whose general structure is illustrated in FIG. 18, the resistance value changes due to a variation in the temperature even in the following control operation, and the correction value used in the operation must be corrected. In the proposed example whose control system process is illustrated in FIG. 19, the load control operation and the unload control operation are only referred to, but influence of the coil resistance in the other control operations is not discussed.
The second problem is that the following control operation is a head positioning operation for controlling the head so as to follow a servo track written in advance on the disk, and the velocity signal used in this operation for velocity feedback control must be a signal that represents the relative velocity of the head with respect to the disk. However, the velocity signal obtained from the counter electromotive voltage signal is a signal that represents the absolute velocity, and therefore, the above-described conventional load control operation is a velocity control operation based on the absolute velocity of the head. Thus, since the conventional control operation is performed based on only the absolute velocity without considering the relative velocity, the velocity feedback that intends to increase the stability involves an error in the conventional control operation. No prior art technique provides a countermeasure to these problems. Further, if the absolute velocity is used in the feedback control operation, the head stops at a target position after it reaches there. Therefore, if a disk track has minute meanders, the head cannot follow the meanders. As a result, a position error is likely to occur.
The third problem is that, when the force disturbance exerted on the actuator is estimated and compensated for based on the velocity signal by an observer, if the relative velocity of the head with respect to the disk is not used, the operation of controlling the head so as to follow tracks in the positioning control itself results in a disturbance vibration. Thus, force disturbance estimation/compensation operation may deteriorate the positioning precision.
The present invention was conceived in view of the above problems. An objective of the present invention is to stably perform positioning of a head without being adversely influenced by friction force resulting from static friction, spindle vibration, or the like.